Lost River

The sign's first word carries the weight. In most years the Big Lost does not flow past Arco at all; Mackay Reservoir upstream and the valley's irrigators take it first, as the laboratory's own hydrologists put it. When it does run, it runs onto the Idaho National Laboratory. A small diversion dam built there in 1958, its channel enlarged in 1984 after repeated flood threats, turns high water into spreading areas so it sinks before reaching the reactor areas. What sinks joins the aquifer and surfaces at Thousand Springs after anywhere from 50 to 700 years.

What the plaque says

Lost River When its water is not diverted for upstream irrigation, Lost River flows past here into a sink 14 miles to the northeast. Lava flows in the Snake River plains buried old channels of Lost River, Little Lost River, and Birch Creek. No longer able to reach the Snake on the surface, they went underground. After a 120 mile journey under the lava plains, water from Lost River eventually emerges through numerous large springs below Twin Falls, making up a small part of the flow of Thousand Springs near Hagerman. 304 Text prepared by the Idaho Historical Society Marker made & installed by Idaho Transportation Department

Where it stands

Worth the stop nearby

• Experimental Breeder Reactor I — 2.6 mi
  The reactor that lit four bulbs in 1951 also suffered the first accidental breeder meltdown, and two engines for a nuclear bomber sit outside.

More markers nearby

• Nuclear Reactors — steps away
• Big Butte — 15 mi
• Three Buttes — 15 mi
• Atoms for Peace — 16 mi